1. Field of the Invention
The present invention relates generally to a hydraulic shock absorber, suitable for use in an automotive suspension system. More specifically, the invention relates to a hydraulic shock absorber which can provide enhanced initial response characteristics.
2. Prior Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 56-119035 discloses one of the typical constructions of shock absorbers which employs a disc valve for resiliently closing the opening end of an essentially axial fluid path defined through a piston. The shock absorber is also provided with a constant orifice through which a working fluid is constantly permitted to flow. The constant orifice is designed for generating damping force at relatively low piston speed range. Namely, in a relatively low piston speed range, the disc valve is held in closed position for blocking fluid communication therethrough because of relatively small pressure difference between upper and lower fluid chambers defined across the piston. In such piston speed range, the constant orifice is only effective for permitting fluid flow and thus for generating damping force by its flow restriction effect. On the other hand, when the piston stroke speed is increased in excess of a critical speed at which the pressure difference between the upper and lower cylinder overcomes the resilient force of the disc valve, the disc valve opens to permit a greater amount of working fluid to flow therethrough. In such a construction substantial variation of the damping characteristics is caused by the transition between a low piston speed range and a high piston speed range changing across a critical speed set forth above.
In order to obtain optimum damping characteristics, it is desirable to provide essentially linear variation characteristics of damping force over all piston speed ranges. For this purpose, it is desirable not to provide a constant orifice so as to provide lineality in variation of the damping characteristics. Furthermore, as discussed in German Patent 833,574, it is preferable to provide damping force variation characteristics proportional in approximately two thirds the power of the piston stroke speed (S.sup.(2/3) : S is piston stroke speed).
Although a certain level of improvement can obtained by removing the constant orifice, it is still not satisfactory for providing satisfactory response characteristics in the generation of the damping force. Particularly, at the initial stage of generating damping force, lack of damping force can be caused by leak flow of the working fluid between the upper and lower working chambers through a small gap defined between the piston and the inner periphery of a shock absorber cylinder or between the upper working chamber and a reservoir chamber through a gap between a piston rod and a guide structure therefor.